Most steel is now made by slab caster, billet caster, bloom caster or thin strip caster, which forms the steel into a semi-finished slab, billet or bloom product or a near-finished strip cast product. Each of these casting processes involves continuous delivery of molten metal to the caster during the casting sequence for full production by the caster. If production is disrupted during the cast in making semi-finished product in a slab, billet or bloom caster, considerable downtime is required to clean or change out the cast mold and place dummy bars to start a new cast. Further, one or more strands of a billet caster can become plugged during the cast slowing the through-put of molten metal by the caster. Although precautions are taken to avoid such disruptions during a cast, such disruptions need to be anticipated and plans set for response when such a disruption occurs. Disruptions in casting of near-finished strip on a roll caster, on the other hand, generally involve simply restarting the cast without downtime or by a rapid change out of the casting rolls and/or refractories and restarting the cast. No cleaning of the mold or placement of a dummy bar is required to restart casting in making strip by roll caster.
In the past, these various casters have been serviced typically by a melt shop employing electric arc furnaces (EAF) or basic oxygen furnaces (BOF) to make hot steel compositions for casting. For slab, billet and bloom casting, the molten metal may be delivered directly to the caster, or delivered through a ladle metallurgy furnace (LMF) where the composition of molten metal from the melt shop is trimmed for the casting operation. Degassing is also less commonly used with slab, billet and bloom casters, but for certain grades of low carbon steel and steel stainless steel (where vacuum-oxygen decarburization (VOD) is typically used) degassing is used for control of the gases in the molten metal composition in preparation for casting. On the other hand, because of the nature of the strip casting process, it has been found generally necessary in all steel grades to control the amount of gases in the molten steel and to trim the composition of the molten metal in an LMF before delivery to a thin strip caster. Unlike slab, billet and bloom casting, continuously casting thin strip into a near-finished product involves forming the basic microstructure of the steel in milliseconds rather than minutes. For this reason, generally the time lapse to prepare the steel composition for casting between delivery of molten metal from a melt shop to strip caster is considerably longer than the time needed to prepare such molten metal for delivery to a slab, billet or bloom caster.
Another difference between making billet, blooms and slab by continuous mold caster and making cast strip by continuous strip caster is the rate of metal through-put. Billet, bloom and slab casters have a relatively limited ability to vary the through-put rate of the mold caster. The exception is in multiple strand mold casters such as the billet caster where one or more strands can become plugged during a cast and an unscheduled through-put of molten metal correspondingly decreased. Otherwise there is less ability to increase or decrease production during a casting campaign than in a continuous strip caster. A continuous strip caster, on the other hand, can substantially increase and decrease molten metal through-put rate by varying casting speed or varying thickness of the cast strip or both. The thinner the strip produced and the faster the strip caster operates, the more molten metal that can be processed by the plant in a given period of time.
As a result, the production of molten metal by the metal shop was generally driven by the needs of the particular caster being serviced. The time between completion of the making of the molten metal and the delivery of the molten metal to the caster have been coordinated so that the molten metal from the furnace on delivery to the caster had sufficient latent heat that the melt would not prematurely cool and disrupt the casting campaign. For this reason, the melt shop typically had more capacity than necessary to service the needs of the serviced caster. Moreover, although the capacity of the melt shop had to take into account the interim ladle treatment requirements for casting operation, the capacity of the melt shop was not matched to the particular need of the caster being serviced except in a gross way. Accordingly, the efficiency of the steelmaking plant was generally below capacity of the melt shop and governed by the through-put of the serviced caster.
The difficulty is compounded by the quite different market demand for, and profitability of, the semi-finished long product from the billet or bloom caster, the semi-finished slab product from the slab caster, and the near-finished strip product from the thin strip casters. In general, the product from the strip caster is more profitable and in higher demand because the product competes with cold roll sheet (which is more expensive to make with the rolling sequences involved). By contrast, semi-finished billets, blooms and slab are more plentiful and typically require further processing to produce a marketable product. Thus, market demand and profit margin of long products and slabs are generally lower than for thin cast strip. Yet, the production demands in making billets, blooms, beam blank, and slabs by continuous casters, with the need to avoid disruption of the casting campaigns, are considerably greater and quite different from the production demands in making thin cast strip.
Disclosed is a steelmaking plant that takes advantage of the full capacity of the melt shop, and produces both finished thin cast strip and semi-finished billets, blooms or slabs. The present steelmaking plant balances the needs and advantages of a strip caster with the operational demands of a billet caster, bloom caster or slab caster to produce both finished and semi-finished steel products in one plant and take use of the full capacity of the melt shop servicing the casters.
Disclosed is a method of making steel comprising the steps of:
(a) assembling a steelmaking furnace capable of melting and making molten metal for delivery to a first metal delivery system and a second metal delivery system,
(b) assembling a thin strip caster capable of continuous casting of steel strip having a thin strip production output, the thin strip caster comprising a pair of casting rolls having a nip there between for delivery of thin strip downwardly there from, and the first metal delivery system capable of providing molten metal forming a casting pool between the casting rolls above the nip with side dams adjacent the ends of the nip to confine the casting pool,
(c) assembling a mold caster capable of continuous casting of steel semi-finished products having a semi-finished production output, the mold caster comprising a casting mold capable of producing one or more strands and having the second metal delivery system capable of introducing molten metal into the casting mold,
(d) inputting to a computer data on raw materials for the steelmaking furnace, the steelmaking furnace availability and capacity for making molten steel, ladle treatment for casting in the thin strip caster and mold caster, thin strip caster and mold caster sequence schedules and through-put, capacities and variability of the thin strip caster and mold caster, and demand and/or customer requirements for thin strip production output and semi-finished production output,
(e) forecasting by processing by the computer from the inputted data a production schedule for the steelmaking furnace and ladle treatment, sequence schedule for the thin strip caster, and sequence schedule for the mold caster as a function of molten metal availability for casting, the thin strip caster and mold caster sequence schedules and through-put, and the demand for thin strip production output and semi-finished production output, and
(f) directing production of the molten metal from the steelmaking furnace and ladle treatment alternatively to the first metal delivery system of the thin strip caster and to the second metal delivery system of the mold caster responsive to said forecasting.
The steps of inputting the data to the computer and forecasting by processing by the computer may be done intermittently during steelmaking. Alternately, the steps of inputting the data to the computer and forecasting by processing by the computer may be done continually during steelmaking.
The steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace takes into account changing the rate of metal delivery through the first metal delivery system and the second metal delivery system during casting.
The steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace may take into account the variable speed of thin strip casting and/or capacity to vary the thickness of cast strip by the strip caster. Alternately or in addition, the steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace may involve steps of determining a desired rate of metal delivery through the first metal delivery system to the strip caster as a function of the molten metal availability and a desired mold caster through-put rate, and selecting caster speed and strip thickness of the thin strip caster corresponding to the determined rate of metal delivery through the first metal delivery system to the strip caster, the determined rate of metal delivery through the second metal delivery system to the mold caster, or both. The ladle treatment may be done separately for the first metal delivery system and the second metal delivery system, or the ladle treatment may be done for the first metal delivery system and not for the second metal system as desired for the particular embodiment.
Specifically, the steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace may include varying casting by the thin strip caster to provide molten metal to the second metal delivery system for continuous casting by the mold caster to avoid disruption of the casting by the mold caster during the casting sequence. Alternatively, the steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace may include varying the mold caster through-put as a function of the molten metal availability and the desired rate of metal delivery through the first metal delivery system to the strip caster.
The steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace may take into account the ladle treatment of the molten steel for casting by ladle metallurgical furnace, degassing the molten metal, or a combination thereof.
The step of forecasting production schedules may include taking into account profitability in making semi-finished production output and thin strip production output. Alternately or in addition, profitability may be a function of customer requirements.
The steps of forecasting by processing by the computer and directing production of the molten metal from the steelmaking furnace may take into account market parameters for semi-finished production output and thin strip production output. The market parameters may include at least one selected from a group consisting of product prices, market indices, market capacity for the products, and orders for semi-finished production output and thin strip production output.